Research On Polarization And Mode Modulation Based On Subwavelength Grating Waveguides

Today,integrated photonic devices,which use light as a signal transmission carrier,have received widespread attention due to their high integration,low power consumption,high speed,and low crosstalk characteristics.The degrees of freedom such as polarization and modes can increase the data capacity in a single physical channel,which is well suited to the development needs of today’s big data industry.In addition,compared to conventional waveguides,subwavelength grating waveguides can provide more flexible refractive index modulation,significantly enhancing the guided mode modulation of optical waveguide devices.Therefore,this paper is mainly based on the structure of the subwavelength grating waveguide,analyzing its mode characteristics,and investigating the corresponding waveguide devices.Firstly,in the mid-infrared band of 2μm,using a silicon-phase change material hybrid subwavelength grating,we realize a polarization beam splitter with dynamically tunable output power;secondly,to address the problem of excessive loss of high-order modes in subwavelength gratings,we propose the concept of a narrow strip-assisted subwavelength grating multimode waveguide and use it to successfully design mode converters and mode multiplexer based on a dual subwavelength waveguide directional coupler;Finally,the flexible dispersion adjustment capability of the brick subwavelength grating are applied to design polarization-insensitive directional couplers and 1×2 optical beam splitters respectively.Details are as follows:（1）Aiming to achieve an ultra-broadband polarization beam splitter with dynamically tunable output power,we have designed the coupling region as a combination of a hybrid sub-wavelength grating waveguide of Si-Ge₂Sb₂Se₄Te₁with a slot waveguide.The subwavelength grating structure provides effective dispersion control for the waveguide device,giving wider optical bandwidths,while the optical phase change material brings tunable transmission.In addition,a bending directional coupler cascaded behind the coupling region can filter out residual TM crosstalk and further broaden the bandwidth.When Ge₂Sb₂Se₄Te₁ is in the amorphous state,the polarization beam splitter works well with an insertion loss of less than 0.24 d B（0.56d B）for TE₀（TM₀）and a polarization extinction ratio greater than 20 d B over an operating bandwidth of 200 nm（130 nm）.When Ge₂Sb₂Se₄Te₁ is switched to another crystal state,the device acts as an energy absorber whose energy decays depending on the state of the phase change material.Additionally,the waveguide thickness is 340 nm,allowing the device to operate in the 2μm band.（2）To achieve efficient manipulation of high-order modes on subwavelength gratings,we propose a narrow-strip-assisted subwavelength grating multimode waveguide that combines the characteristics of subwavelength grating waveguides and strip waveguides,possessing good dispersion modulation capability and,at the same time,can effectively support higher-order modes.Based on this,we propose and design a dual-subwavelength-waveguide directional coupler-based mode converter,whose coupling region consists of a brick subwavelength grating waveguide and a narrow-strip-assisted subwavelength grating multimode waveguide,enabling TE₀-TE₁ and TE₀-TE₂ mode conversion.We have also designed a three-channel mode multiplexer（TE₀/TE₁/TE₂）with a wider operating bandwidth（136 nm/135 nm/100 nm）,low insertion loss（0.29 d B/0.85 d B/0.62 d B）,and low crosstalk（-21.4 d B/20.4 d B/18.0 d B）.In addition,the narrow strip-assisted subwavelength grating multimode waveguide shows excellent scalability,such as mode conversion for TE₀-TE₄,and efficient transmission for TE₇.（3）To solve the problem of polarization sensitivity inherent to silicon-based optical waveguide devices,we have designed polarization-insensitive directional couplers and 1×2 optical beam splitters by exploiting the more flexible dispersion regulation capability of brick subwavelength grating waveguides and modulating the waveguide refractive index from two different dimensions.They have equal coupling lengths for TE polarization and TM polarization.For the polarization-insensitive directional coupler,the normalized output power is 0.935 for TE polarization and 0.933for TM polarization at a wavelength of 1.55μm.For the polarization-insensitive 1×2beam splitter,the insertion loss and splitting ratios are 0.7 d B and 1.000（0.58 d B and1.0009）for TE polarization（TM polarization）at a wavelength of 1.55μm,respectively.In the designing of optical waveguide devices,various types of subwavelength grating structures can be used to achieve flexible modulation of the polarization,mode,and wavelength of the guide modes in optical waveguides,and then design large bandwidth,low loss,and ultra-compact waveguide devices,which is of great positive significance for the development of integrated photonic technologies such as optical interconnection,optical sensing,and optical communication.